1. Field of the Invention
The present invention is directed to self-assembled nanoparticle arrays, methods of making the nanoparticle arrays, and methods of using the nanoparticle arrays in spectroscopic methods for detecting targets of interest.
2. Background
Plasmonic structures are routinely used in Raman spectroscopy to increase the cross-section for the scattering process and augment the signal, particularly when very little sample is available, a technique termed surface-enhanced Raman scattering (SERS) (Brown, R. J. C., et al., J. Raman Spectrosc. 39:1313 (2008); Camden, J. P., et al., Acc. Chem. Res. 41:1653 (2008)). The extreme sensitivity of SERS is achieved through the strong coupling between electromagnetic radiation, plasmon modes of a surface, and electronic states of a molecule, which in turn couple to the vibrational modes of the molecule (J. Gersten and A. Nitzan, J. Chem. Phys. 73:3023 (1980); Schatz, G., et al., in Surface-Enhanced Raman Scattering—Physics and Applications, Topics Appl. Phys. 103:19 (2006)). The SERS substrate, containing the plasmonic structure and often a binding site for the molecule to be analyzed, needs to be tailored precisely (i.e., at the nanoscale) in order to achieve efficient coupling and provide colossal signal enhancement (Ko, H., et al., Small 4:1576 (2008)). In many cases, it is desirable to design the SERS substrate so that its surface plasmon resonance frequency lies between the frequencies of the incident light (the laser of the Raman spectrometer) and the scattered light (McFarland, A. D., et al., J. Phys. Chem. B 109:11279 (2005); Chu, Y. Z., et al., ACS Nano 4:2804 (2010); Felidj, N., et al., Appl. Phys. Lett. 82:3095 (2003)). The surface plasmon resonance frequency may also need to match approximately an electronic transition energy of the molecule or solid being probed to make use of the resonant Raman scattering effect and further increase the enhancement factor (Zhao, J., et al., J. Phys. Chem. C 112:19302 (2008)). Self-assembly techniques (Freeman, R. G., et al., Science 267:1629 (1995); Wang, H., et al., J. Am. Chem. Soc. 127:14992 (2005); Wang, H., et al., Angew. Chem. Int. Ed 46:9040 (2007); Wei, A., et al., Chem Phys Chem 2:743 (2001); Mu, C., et al., Nanotechnology 21:015604 (2010); Dick, L. A., et al., J. Phys. Chem. B 106:853 (2001); Baumberg, J. J., et al., Nano Lett. 5:2262 (2005)), nano-lithography techniques (Felidj, N., et al., Appl. Phys. Lett. 82:3095 (2003); Hatab, N. A., et al., Nano Lett. 7:4952 (2010); Fromm, D. P., et al., J. Chem. Phys. 124:061101 (2006); Ward, D. R., et al., Nano Lett. 7:1396 (2007)), and combinations thereof (Alexander, K. D., et al., J. Raman Spectrosc. 40:2171 (2009); Yan, B., et al., ACS Nano 3:1190 (2009); Stoerzinger, K. A., et al., J. Phys. Chem. Lett. 1:1046 (2010); Lee, S. Y., et al., ACS Nano 4:5763 (2010)) have been employed to fabricate and optimize SERS substrates for the detection of and discrimination between analytes. Each fabrication method for SERS substrates involves a compromise between enhancement factor, cost, active area, reproducibility, and service life.
Nanoparticles show unique optical, magnetic, and electric properties, which are often size or shape dependent and different from the properties of the respective bulk material (R. Shenhar and V. M. Rotello, Acc. Chem. Res. 36:549 (2003); Shipway, A. N., et al., Chem Phys Chem. 1:18 (2000); Nie, Z. H., et al., Nature Nanotechnol. 5:15 (2010); Eychmuller, A., J. Phys. Chem. B 104:6514 (2000); C. N. R. Rao and A. K. Cheetham, J. Mater. Chem. 11:2887 (2001)). As these properties often only emerge when appropriate phenomena of coupling and exchange between the nanoparticles exist, the nanoparticle position distribution generally needs to be controlled through immobilization and assembly of the nanoparticles on a substrate or in a medium. Recently, various polymer/nanoparticle hierarchical structures have been introduced as candidates for use in next generation applications in electronic devices and sensors (Chiu, J. J., et al., J. Am. Chem. Soc. 127:5036 (2005); B. H. Sohn and B. H. Seo, Chem. Mater. 13:1752 (2001); Lin, Y., et al., Nature 434:55 (2005)). The use of block copolymers to produce nanoscale templates has gained increasing attention as the block copolymer morphology is determined by the volume fraction of the polymer blocks, and the size and the distance between domains is determined by the overall molecular weight (F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem. 41:525 (1990)). Thus, block copolymers can be utilized as templates for controlling the spatial location of nanoparticles (S. B. Darling, Prog. Polym. Sci. 32:1152 (2007); Darling, S. B., et al., Adv. Mater. 17:2446 (2005); S. B. Darling, Surf Sci. 601:2555 (2007)). Among the processing techniques available for directing the self-assembly of block copolymer thin films, controlling the rate of solvent evaporation has attracted particular attention as it is simple and generally not sensitive to the substrate. An example is provided by polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer having cylindrical microdomains of PEO. After solvent annealing, defect-free lateral ordering of vertically oriented PEO cylinders can be achieved over several micrometers (Kim, S. H., et al., Adv. Mater. 16:226 (2004)). Kim et al. have reported a simple route for fabricating a nanopatterned array of inorganic oxide semiconductors using the PS-b-PEO film as a template (Kim, D. H., et al., Nano Lett. 4:1841 (2004)). In the film, the PEO forms hexagonally ordered domains with a 2 nm depression in each of the PEO domains, where semiconductors such as silica and titania were grown by exposure to precursor vapor. Polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) has been used to synthesize nanoparticles by exploiting specific interactions between the P4VP block and metallic precursors (M. Aizawa and J. M. Buriak, Chem. Mater. 19:5090 (2007); Glass, R., et al., Adv. Funct. Mater. 13:569 (2003); Lohmueller, T., et al., Adv. Mater. 20:2297 (2008); Yun, S. H., et al., Langmuir 21:6548 (2005)). Typically, metallic precursor loaded PS-b-P4VP micelles are deposited onto a substrate by spin-coating or dip coating. Subsequent plasma treatment converts the precursors to nanoparticles and eliminates the polymer; the result is a hexagonal array of nanoparticles that matches the micellar monolayer present before plasma treatment. Metal nanoparticles have been widely used as catalysts for the growth of nanowires and nanotubes (Chan, C. K., et al., Nature Nanotechnol. 3:31 (2008); Huang, J. X., et al., Nano Lett. 6:524 (2006); Lee., D. H., et al., Adv. Mater. 20:2480 (2008); Lu., J., et al., J. Phys. Chem. B 110:6655 (2006)), in sensors using surface enhanced Raman scattering (Freeman, R. G., et al., Science 267:1629 (1995); Yan, B., et al., ACS Nano 3:1190 (2009)), and for optoelectronics (Banerjee, P., et al., ACS Nano 4:1019 (2010)). However, as there was no covalent or electrostatic interaction between the plasma generated metal nanoparticles and the substrate, nanoparticle detachment from the substrate leads to the loss of the pattern (Lohmueller, T., et al., Adv. Mater. 20:2297 (2008)).
The present invention provides a fabrication method for nanoparticle arrays with tunable nanoparticles size and tunable separation between adjacent nanoparticles. The method applies to all nanoparticles that can be immobilized on at least one of the polymer blocks. Therefore, nanoparticles with useful electrical, optical, magnetic, chemical and catalytic properties can be used to form the arrays.
The present invention provides a fabrication method for SERS substrates that possess a unique combination of three highly desirable attributes: (a) the SERS substrates can be tuned to match the laser wavelength of operation and maximize the enhancement factor for the particular Raman instrument and analyte in use; (b) the SERS substrates have a highly reproducible enhancement factor over macroscopic sampling areas; and (c) the fabrication method is achieved without resorting to expensive, slow nano-lithography tools. The substrates are made entirely through self-assembly and templating techniques, which are cost-effective and scalable to large areas. These attributes make the fabrication process very appealing for mass production. Similarly, the present invention provides a fabrication method for substrates for other surface enhanced spectroscopic techniques, such as surface enhanced fluorescence, surface enhance (infrared) absorption, surface enhance (Raman) optical activity, and surface enhanced circular dichroism.
The present invention provides a fabrication method for magnetic media substrates for information storage. The magnetic nanoparticles arrays can be produced with densities of at least 2×1011 nanoparticles/cm2 uniformly over large areas. The substrates are made entirely through self-assembly and templating techniques, which are cost-effective and scalable to large areas.